If you're ready to subclass LibRaw to own class to access protected fields (libraw_internal_data.unpacker_data) , use this method:
position to libraw_internal_data.unpacker_data.data_offset and read header (16 bytes), data version in 3rd byte of header (header[2]), it is 1 for lossless compression (supported) and 0 for unsupported lossy.
There is no direct/accurate way in LibRaw 0.20
If you're ready to subclass LibRaw to own class to access protected fields (libraw_internal_data.unpacker_data) , use this method:
position to libraw_internal_data.unpacker_data.data_offset and read header (16 bytes), data version in 3rd byte of header (header[2]), it is 1 for lossless compression (supported) and 0 for unsupported lossy.